This invention relates to a method and apparatus for reflective ultrasonic imaging in medical and industrial applications. More particularly, the invention relates to an ultrasonic scanner with associated circuitry and to a method for generating acoustic impedance projections from reflection data in a computerized reconstruction tomography system.
Conventional ultrasonic imaging systems including those operating in B-scan mode do not provide quantitative information pertaining to a bulk, constitutively significant property of the imaged specimen or object. The deficiencies in such systems result mainly from direct utilization of the echo amplitude as the imaged quantity. The amplitude in turn is modulated in a complicated fashion by reflection, refraction, diffuse diffractive scattering, and bulk attenuation. Thus, the images portray interfacial geometry but cannot be quantified in terms of any simple bulk specimen property.
It is, however, desirable in both medical and nondestructive testing environments to have an accurate, quantitative measure of the bulk specimen properties. This desirability has been borne out amply by recent experience in medical x-ray tomography. Time-of-flight (TOF) computerized tomography is one ultrasonic technique which provides quantitative reconstructions of acoustic velocity distributions. For further information, reference may be made to copending application Ser. No. 716,109 filed on Aug. 20, 1976 by Gary H. Glover, now U.S. Pat. No. 4,075,883 entitled "Ultrasonic Fan Beam Scanner for Computerized Time-of-Flight Tomography", and assigned to the same assignee as this invention. The system generates numeric values indicative of the acoustic velocity at known coordinates in a specimen layer, which can be displayed in numeric printout form or as a video image by gray scale or pseudo-color encoding the various velocity values. In noninvasive breast examination, tumorous tissue is readily distinguished from normal tissue on the basis of the computed velocity values. However, time-of-flight tomography is limited to only these few portions of the anatomy which allow transmission through the specimen from a number of angles.
The principal object of the present invention is to realize an ultrasonic imaging system and method based on reflection which yields quantitative data, to thus combine the advantages of reflection systems (one-port viewing) with computerized transmission systems (quantitative results).